Halogenoalkanes

• Halogenoalkanes


  Alkanes with one or more halogen atoms (F, Cl, Br, I) attached to the carbon chain
• Halogen prefixes


  • Fluoro for F
  • Chloro for Cl
  • Bromo for Br
  • Iodo for I
• Halogenoalkanes


  • 1-bromopropane
  • 2-chloro-2-methylbutane
• Primary halogenoalkane


  • One carbon attached to the carbon atom adjoining the halogen
• Secondary halogenoalkane


  • Two carbons attached to the carbon atom adjoining the halogen
• Tertiary halogenoalkane


  • Three carbons attached to the carbon atom adjoining the halogen
• Nucleophilic substitution reactions of halogenoalkanes

  1. Nucleophile attacks positive carbon

  2. Halogen leaves as X-

  3. New group bonds to carbon
• Nucleophile


  Electron pair donator e.g. :OH-, :NH3, CN-
• Halogenoalkanes undergo either substitution or elimination reactions
• Strength of C-X bond

  Determines rate of substitution reaction
• Bond enthalpies


  • C-I: 238 kJ/mol
  • C-Br: 276 kJ/mol
  • C-Cl: 338 kJ/mol
  • C-F: 484 kJ/mol
• Iodoalkanes are fastest to substitute, fluoroalkanes are slowest
• Nucleophilic substitution with aqueous hydroxide ions

  Halogenoalkane + KOH -> Alcohol + KX
• Nucleophilic substitution with cyanide ions

  Halogenoalkane + KCN -> Nitrile + KX
• Nucleophilic substitution with ammonia

  Halogenoalkane + 2NH3 -> Amine + NH4X
• Naming amines


  Use IUPAC names e.g. propan-1-amine
• Naming nitriles


  Nitrile group at end of chain, number from C in CN
• Elimination reaction with alcoholic hydroxide ions

  Halogenoalkane + KOH -> Alkene + KX + H2O
• Tertiary halogenoalkanes tend towards elimination, primary towards substitution
• Ozone in upper atmosphere is beneficial, in lower atmosphere is a pollutant
• CFCs caused hole in ozone layer
• Ozone depletion by CFCs

  1. UV breaks C-Cl bond in CFC

  2. Cl radical catalyses decomposition of O3 to O2
• Legislation banned CFCs, HFCs developed as safer alternative
• Uses of halogenoalkanes


  • Solvents
  • Refrigerants
  • Pesticides
  • Aerosol propellants
• Many uses of halogenoalkanes stopped due to toxicity and environmental impact